1. Field of the Invention
The present invention relates to a polycarbonate resin composition, and more specifically to a polycarbonate resin composition excellent in flame resistance, impact resistance, hue, and color fastness, and a molded article obtained by molding the same.
2. Description of the Related Art
Polycarbonate resin is excellent in heat resistance, mechanical performances, and electrical characteristics, and has widely been used for example in automotive materials, materials of electric/electronic instruments, housing materials, and materials for manufacturing components in other industrial fields. In particular, flame retardant polycarbonate resin composition is preferably used as components of OA and information appliances such as computer, notebook-sized personal computer, mobile phone, printer, and copying machine.
The polycarbonate resin has been given flame resistance by adding thereto a halogen-containing flame retardant or phosphorus-containing flame retardant.
A known example of the polycarbonate resin composition added with a phosphorus-containing flame retardant is disclosed in Patent Document 1. The technique of adding a phosphorus-containing flame retardant to the polycarbonate resin might have been successful in achieving a high level of flame resistance and an excellent fluidity, but has been suffering from considerable degradation in the impact resistance which polycarbonate resin inherently possesses.
There have therefore been vigorous efforts of improving the impact resistance by further adding an elastomer. Among others, the impact resistance may be improved effectively by using a graft copolymer composed of a rubber-like polymer and a monomer polymerizable therewith, and more preferably a core/shell type graft copolymer composed of a core made of a rubber-like polymer and a shell made of a polymerizable monomer. (See Patent Document 2, for example).
Other proposals relate to resin compositions which contain a graft copolymer having a core made of a polyorganosiloxane-butyl acrylate-based rubber and a shell made of acrylonitrile and styrene, and having an average particle size of 0.1 to 0.2 μm; or a graft copolymer having a core made of butadiene-unsaturated carboxylic acid alkyl ester-aromatic vinyl-based rubber and a shell made of styrene and methyl acrylate, and having an average particle size of 100 to 180 nm (see Patent Documents 3 to 5).
These sorts of graft copolymers are, however, readily combustible, so that the polycarbonate resins added with the phosphorus-containing flame retardant, and further with the graft copolymer, were considerably degraded in the flame resistance.
In particular, since the graft copolymer is generally known to be more combustible as compared with the polycarbonate resin, so that the composition which contains the polycarbonate resin together with the graft copolymer has been suffering from considerable increase in heat generation rate. This means not only that the composition violently combusts when ignited and highly endangers people therearound, but also that the composition is likely to spread fire in case of fire accident. It is therefore a strong social need for material having an excellent flame resistance conforming to UL94, and a small heat generation rate.
On the other hand, recent vigorous investigations has been directed to metal organosulfonate compound represented by alkali metal organosulfonate compound and organoalkaline earth metal salt compound, as useful flame retardants (see Patent Documents 6 and 7, for example).
Also for this case, further addition of an elastomer has been discussed aiming at improving the impact resistance (see Patent Document 8, for example). Among others, diene-based rubber has been used as the elastomer, by virtue of a good balance between impact, resistance and flame resistance.
The diene-based rubber is, however, readily colored due to oxidative degradation. The above-described resin compositions having been used were poor in hue, and were more likely to cause yellowing, when they were molded and used as components which are succeedingly exposed to heat.
The resin compositions were also likely to cause yellowing even in pre-drying before molding. In order to obtain the molding with a good hue, it has therefore been necessary to strictly control the drying time, which has considerably degraded the continuous productivity in practice.
Moreover, the above-described graft copolymers having been used were likely to discolor due to heat in the process of pellet making or molding, so that the above-described resin compositions have often been suffering from poor hue, and also from tendency of discoloration due to prolonged exposure to heat after being molded.